In a Rose Garden announcement, President Obama said that he “could choose no manager as expert, as competent” as Ms. Mathews.

The announcement comes a day after public disclosure of the resignation of Ms. Sebelius, who during her five-year tenure had come in for widespread Republican criticism that increased sharply after the rocky rollout of HealthCare.gov last fall.

Ms. Burwell last year was confirmed 96-0 for her post as head of OMB. Her background includes work in the Clinton administration and at the Bill & Melinda Gates Foundation.

Testifying before Congress April 10, Ms. Sebelius said that even with the formal registration period ended, enrollment in public health insurance exchanges continues to grow and now tops 7.5 million.

“Last week, we announced that 7.1 million Americans have signed up” for exchange coverage, she told the Senate Finance Committee.

“As of this week, 400,000 additional Americans have signed up—and we expect that number to continue to grow,” Ms. Sebelius added.

Enrollment growth continues because many of the exchanges — authorized by Patient Protection and Affordable Care Act — agreed to accept enrollees after the March 31 enrollment deadline as long as individuals attempted but were unable to complete their enrollments because of technical issues, by that date.

For example, California, the largest exchange with more than 1.2 million enrollees as of March 31, said those who opened an online account and completed the first page of the application by March 31 will have until April 15 to complete their applications and obtain coverage.

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This report appeared on modernhealthcare.com, the website of Modern Healthcare magazine, a Chicago-based sister publication of Tire Business.